Spinel

The spinels are any of a class of chalcogenides.

Types of Spinel

Important members of the spinel group include:

• Spinel – MgAl₂O₄, after which this class of minerals is named
• Gahnite - ZnAl₂O₄
• Franklinite - (Fe,Mn,Zn)(Fe,Mn)₂O₄
• Chromite - (Fe·Mg)Cr₂O₄
• Magnetite - Fe₃O₄
• Hercynite - FeAl₂O₄
• Ulvöspinel - TiFe₂O₄
• Jacobsite - MnFe₂O₄
• Trevorite - NiFe₂O₄
• polymorph within the Earth's mantle from about 520 to 660 km depth, and a rare mineral in meteorites

Properties of true spinel

Spinel crystallizes in the isometric system; common crystal forms are octahedra, usually twinned. It has an imperfect octahedral cleavage and a conchoidal fracture. Its Samarian Spinel is the largest known 500 carat spinel in the world.

The transparent red spinels are called spinel-rubies or balas-rubies and were often confused with actual rubies in ancient times. "Balas" is derived from Balascia, the ancient name for Badakhshan, a region in central Asia situated in the upper valley of the Kokcha river, one of the principal tributaries of the Oxus river.

Occurrence

True spinel has long been found in the ruby are often found together.

Spinel, (Mg,Fe)(Al,Cr)₂O₄, is common in plagioclase is the more stable aluminous mineral in peridotite.

Spinel, (Mg,Fe)Al₂O₄, is a common mineral in the Ca-Al-rich inclusions (CAIs) in some chondritic meteorites.

The Spinel structure

In the so-called normal spinel structure, X cations occupy the tetrahedral sites of the oxide lattice, and Y cations the octahedral sites. For inverse spinels, half the Y cations occupy the tetrahedral sites, and both X and Y cations occupy the octahedral sites. For many years, σ-type interactions between the metal cations and the oxide anions. This rationale can explain anomalies in the spinel structures that crystal-field theory cannot, such as the marked preference of Al³⁺ cations for octahedral sites or of Zn²⁺ for tetrahedral sites - using crystal field theory would predict that both have no site preference. Only in cases where this size-based approach indicates no preference for one structure over another do crystal field effects make any difference - in effect they are just a small perturbation that can sometimes make a difference, but which often do not.

References

1. ^ J.K. Burdett, G.L. Price and S.L. Price, J. Am. Chem. Soc., 104 (1982), 92-95

See also

• Deer, Howie and Zussman (1966) An Introduction to the Rock Forming Minerals, Longman, pp.424-433, ISBN 0-582-44210-9
• Shumann, Walter (2006) Gemstones of the World 3rd edition, Sterling, pp.116-117.
• Spinel structure by Steven Dutch
• Spinel structure
• http://www.gemstone.org/gem-by-gem/english/spinel.html
• http://mineral.galleries.com/minerals/oxides/spinel/spinel.htm
• http://www.mindat.org/min-3729.html
• http://www.webmineral.com/data/Spinel.shtml
• The story of the worlds most famous Red Spinel, the Black Princes Ruby